This invention involves polyurethane foams, preferably such foams having viscoelastic properties.
Foamed articles, especially polyurethane foamed articles are known for many years and used in many applications, especially for cushioning and mattresses. Resilient and viscoelastic or “memory foams” are among the foams known in the art. The viscoelastic foams can be produced by a number of different chemical approaches, all involving reactions of polyol and isocyanate compounds. The products are usually foamed using polyols having a functionality of an average of about 3 hydroxyl groups per molecule and a molecular weight range of about 400-1500. The foams are produced at low isocyanate index as compared to other polyurethane foams, below about 95, often 60-95, typically using water levels at most about 2.5 parts by weight per 100 parts by weight polyol (pphp), often in the range of 0.8-1.5 pphp, an usually with special silicone surfactants to control cell structure. The silicone surfactants assist in the cell opening process as well as control the overall structure of polyurethane cells. Formulation at a low isocyanate index also results in a foam with an improved level of softness and feel for bedding applications. Since the available isocyanate is limited or “under indexed,” this promotes a competition for these molecules between the water and the polyol system thereby making the formulation technique very different from conventional urethanes. The low water level relative to common slab foams results in more polyol hydroxy groups available for reaction with isocyanate groups and less water for blowing, thus a more dense foam.
It would be desirable to use higher water levels while retaining viscoelastic properties because higher water levels result in lower density foam, and higher foam hardness (higher load bearing) at equivalent foam density. It would be desirable to use a higher isocyanate index without sacrificing viscoelastic properties because higher isocyanate levels help in reducing the residual toluenediamine (TDA) levels in the foam which result from the reaction of toluene diisocyanate (TDI) with two equivalents of water.
These viscoelastic polyurethane foams made from conventional polyols, specifically polyether polyols and isocyanates generally have undesirably low air flow properties, even at their best, less than about 1.0 standard cubic feet per minute (scfm) (0.47 l/s) under conditions of room temperature (22° C.) and atmospheric pressure (1 atm), therefore promote sweating when used as comfort foams (for instance, bedding, seating and other cushioning). Poor airflow also leads to poor heat and moisture conduction out of the foam resulting in (1) increased foam (bed) temperature and (2) moisture level. The consequence of higher temperature is higher resiliency and lowered viscoelastic character. Combined heat and moisture result in accelerated fatigue of the foam (bed). In addition, if foam air flows are sufficiently low, foams can suffer from shrinkage during manufacturing. Furthermore, improving the support factor of viscoelastic foams made from conventional polyether polyols is limited unless viscoelastic properties are compromised. These disadvantages are sometimes addressed by addition of copolymer polyols such as those containing styrene/acrylonitrile (SAN), for instance as taught in “Dow Polyurethanes Flexible Foams,” 2nd edition, R. L. Herrington and K. Hock, ed. (1997). In addition to their complicated manufacture (resulting in high costs) the SAN copolymer polyols offer disadvantages of increasing the resiliency and decreasing the recovery time of the foam products, which degrade the desirable properties of viscoelastic foams.
It would be desirable to achieve a higher air flow value than is now achieved using a conventional polyether polyol while retaining viscoelastic properties of a foam, preferably an air flow greater than 1.0 scfm (0.47) l/s under conditions of 22° C. and 1 atm of pressure, preferably in the substantial absence of copolymer polyols especially those containing styrene/acrylonitrile polymers. Furthermore, it would be desirable to do so without sacrificing support factor, more preferably while improving support factor. Support factor is believed to be indicative of longer term durability of the foamed product.